1. Field of the Invention
The present invention relates to a color identification method for color image forming apparatuses such as a color printer and a color copier in an electrophotographic method, an ink jet method or the like, and an image forming apparatus for controlling an image processing portion by using information obtained by this color identification method.
2. Related Background Art
In recent years, higher quality of output images is required for color image forming apparatuses such as a color printer and a color copier adopting an electrophotographic method, an ink jet method or the like.
In particular, gradation in density and stability thereof have a great influence on judgement made by a human being as to whether an image is good or not.
As for the above described color image forming apparatuses, however, the density of the obtained image changes if portions of the apparatus change due to environmental change and long-time use.
In particular, in the case of the color image forming apparatus in the electrophotographic method, even a slight change in the density may cause a color balance to collapse so that it is necessary to always keep constant density-gradation characteristics.
Accordingly, for toner of each color, it has process conditions such as several kinds of exposure amounts and development biases according to absolute humidity and gradation correcting means such as a lookup table (LUT), and selects the process conditions at the time and an optimum value of gradation correction are selected on the basis of the absolute humidity measured by a temperature/humidity sensor.
In addition, it has a configuration wherein, to obtain the constant density-gradation characteristics even if change of each part of the apparatus takes place, a density detecting toner patch is created with the toner in each color on an intermediate transfer member, a drum or the like, and the density of the unfixed toner patch is detected by an unfixed toner density detecting sensor, so that density control is performed by feeding back the process conditions such as the exposure values and development bias from the detection results so as to obtain stable images.
However, in the density control using the unfixed toner density detecting sensor, the patch is formed on the intermediate transfer member, a drum or the like to detect the density, and control is not carried out on transfer to a transfer medium and change in the color balance of the image due to fixing.
The color balance also changes due to transfer efficiency in transferring a toner image to the transfer medium and due to heating and pressurization for fixing. This change cannot be handled by the density control using the unfixed toner density detecting sensor. Accordingly, an image forming apparatus has been devised which has a density or chromaticity detecting sensor for detecting the density of a monochromatic toner image or the chromaticity of a full color image on the transfer medium after the transfer and fixing (hereafter, referred to as a color sensor), forms a density or chromaticity controlling color toner patch (hereafter, referred to as a patch) on the transfer medium, feeds back the detected density or chromaticity to the process conditions such as the exposure amount, and lookup table (LUT), and performs density or chromaticity control of a final output image formed on the transfer medium.
In order to identify CMYK and detect the density or chromaticity, for instance, this color sensor uses light sources emitting light in red (R), green (G) and blue (B) as a light emitting device, or uses a light source emitting light in white (W) as a light emitting device and three types of filters which are formed on a photoreceptor and are different in spectral transmittance of red (R), green (G), blue (B), etc. It is possible to identify the CMYK and detect the density from the three different outputs obtained, for example, RGB outputs.
In addition, it is possible to mathematically process the RGB outputs by linear transformation and so on or convert it by the lookup table (LUT) so as to detect the chromaticity.
Also in a printer in the ink jet method, the color balance also changes due to a change in ink discharge over time, an environmental difference and individual differences in ink cartridges so that the density-gradation characteristics cannot be kept constant. Accordingly, it is devised to install the color sensor in the vicinity of an output portion of the printer so as to detect the density or chromaticity of the patch on the transfer medium and perform the density or chromaticity control.
There are various methods of controlling the density or chromaticity. For instance, a gamma characteristic is controlled from the measured density, or a color matching table and a color separation table are corrected from the measured chromaticity.
In order to detect the absolute density or the absolute chromaticity of the patch by using the color sensor, however, for the following reasons, a criterion is required in which the absolute value of the density or chromaticity of a sensor output proofreading white reference board or the like is known.
The first reason is that it is necessary to proofread variations in spectral characteristics of the light emitting devices and photoreceptors constituting the sensor.
The second reason is that even when the same patch is detected, the outputs may be different due to change over time in the light emitting portion and the photoreceptive portion of the sensor and change in ambient temperature.
The third reason is that a lot of transfer mediums pass through the vicinity of the sensor during normal printing, and so paper powder, toner or ink is scattered to accumulate or stick on the sensor surface, bringing about reduction in sensor output.
However, the white reference board which is often used as a criterion of sensor output proofreading is not only expensive but also possibly gets paper powder, toner or ink scattered thereon as in the sensor to become no longer usable as a reference board.
On the other hand, if the density or chromaticity of the patch is detected without using the criterion for the sensor output proofreading, that is, without proofreading the sensor output, the sensor outputs a value different from actual density or chromaticity of the patch when affected by the above reason.
If the density or chromaticity is controlled by using the results, no color balance is achieved and desired density-gradation characteristics cannot be obtained. In addition, in some cases, the color balance is further lost and the density-gradation characteristics deteriorate.